Synthetic array processor

ABSTRACT

A synthetic array processor wherein radar data, received from an area to be mapped during a plurality of subarray flight path segments, is electronically focused in parallel processing channels to form a series of approximately rectangularly shaped low azimuth resolution maps; and signals associated with corresponding portions of each low resolution map are further processed by means of digital filtering techniques to provide, in a format readily adapted for display, a high azimuth resolution map.

BACKGROUND OF THE INVENTION

This invention relates generally to synthetic array mapping processors,and more particularly to a radar telescope for providing high resolutionimagery of relatively large areas at short ranges.

In synthetic array mapping, radar data received from selected rangeresolution elements within an illuminating beam is periodically sampledas the antenna is moved along a flight path. This data is thenelectronically focused to simulate the physical focus (narrow azimuthbeam width) of an antenna having a length approximately equal to theflight path segment over which the synthetic array was formed.Basically, the electronic focusing involves amplitude weighting andphase adjustment of a sequence of range gated radar returns to cause thereturns from a particular path of ground to be accentuated while thosefrom other contiguous ground patches are attenuated. Due to aircraftmotion during the time required to collect the data, a phase history isimpressed on the returns from each target; and for these returns to addin phase requires that phase compensation be applied thereto.

One processing technique, sometimes referred to as "batch" processing,applies a phase adjustment which "tracks out" the doppler frequency ofthe center point of the map area. Filter banks, responsive to thedoppler frequency differential across the target area, are then utilizedto provide signals indicative of the radar reflectivity characteristicsof the mapped area.

Another processing technique, sometimes referred to as "line-by-line"processing, accomplishes the required phase correction by applying theproper phase adjustments to a sequence of range gated radar returns sothat they focus on a particular ground point. The sequence is thenadvanced one range resolution element across the aircraft track (rangedimension) and the proper phase adjustments reapplied. This process isrepeated until all range elements across the swath have been operatedupon, at which time the sequence starts back at the first range elementand advances one azimuth element along the aircraft track (azimuthdimension). In some applications parallel processing channels are usedso that the different azimuth elements are processed simultaneously foreach range interval.

A significant advantage of "batch" processing is that it is particularlywell adapted to digital implementations; and with innovations such asthe Fast Fourier Transforms techniques (sometimes referred to as the"Cooley-Tukey algorithm") the number of mathematical operations requiredto generate a block of N azimuth resolution elements is reduced from N²for line-by-line processing to 2N log ₂ N for the batch processingtechnique. Batch processors have, however, severaldisadvantages--particularly for reconnaissance applications. One ofthese is related to the geometry of batch processing inasmuch as forreasonably simple mechanizations, the angular and not the azimuthresolution is constant across a block of data. For this reason the blockof data tends to have a keystone shape which makes it very difficult togenerate a composite map by fitting separate blocks together; and theadditional display processing caused thereby increases the complexity of"batch" type processor systems.

SUMMARY OF THE INVENTION

A primary object of the subject invention is to provide an improvedmethod and apparatus for processing, with a reduction in the number ofrequired arithmetic operations, radar data to yield high resolutionsynthetic array imagery.

Another object is to provide a synthetic array mapping system which ispractical to implement for high resolution applications; which providesimagery in a format readily adapted for display; and which minimizesalignment problems between adjacent subsections of the map.

A further object is to provide a radar telescope mechanism which makespractical from an equipment complexity standpoint, high resolutionmapping of a relatively large area at short ranges.

Yet another object is to provide a radar telescope which allows highresolution mapping of relatively large areas at short ranges and over awide range of "look angles", and which avoids display registrationproblems.

In accordance with the subject invention, the display flexibility of"line-by-line" processing is combined with the reduction in equipmentcomplexity feature of "batch" processing. These advantages are realizedby a novel technique wherein low azimuth resolution maps of a selectedarea are formed by line-by-line processing the received data from eachof a plurality of subarrays; and then processing signals fromcorresponding portions of each low azimuth resolution map by means offurther digital filtering techniques to provide a high resolution maptherefrom. The method and apparatus in accordance with the subjectinvention provide map shape flexibility as a result of the approximatelyrectangular layout of the low resolution maps formed by the line-by-lineprocessing technique. The keystone inaccuracies associated with batchprocessing are minimized, as they apply only within the relatively smallresolution blocks formed by the line-by-line processing, instead of tothe whole map. Overall system efficiency results from the fact themaximum rate of arithmetic operations is determined primarily by thereduction from coarse resolution to fine resolution, and in accordancewith the invention this step may utilize the more efficient spectrumanalysis techniques, such as the Fast Fourier Transform algorithm, forexample.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of this invention, as well as the invention itself,will be better understood from the following description taken inconjunction with the accompanying drawings in which like referencecharacters refer to like or similar parts and in which:

FIGS. 1, 1a, 2 and 3 illustrate the flight path--terrain relationship ofthe data processed by one preferred embodiment of the subject invention,operating in a radar telescope mode;

FIG. 4 is a block diagram of a radar mapping system which includes asynthetic array processor in accordance with the subject invention;

FIG. 5 is a block diagram of a presum unit suitable for incorporationinto the system of FIG. 4;

FIG. 6 is a block diagram of one preferred embodiment of a syntheticarray processor in accordance with the subject invention;

FIG. 7 is a more detailed block diagram of one channel of the processorof FIG. 6;

FIG. 8 is a diagram of flight path-terrain geometry useful forexplaining the method of computating focusing coefficients for theprocessor of FIGS. 6 and 7; and

FIGS. 9a, 9b and 9c are timing diagrams helpful for explaining theoperation of the processor of FIGS. 6 and 7.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As shown in FIG. 1, an aircraft 10 is assumed to fly a straight lineflight path 12, and an antenna 14 illuminates a section of terrain whichincludes an area 16 to be mapped. Received signals reflected fromdiscrete scatterers within the real antenna beam 18 are periodicallysampled at range intervals corresponding to area 16. The synthetic arrayprocessor in accordance with the subject invention, electronicallyfocuses the received radar data to simulate the physical focus (narrowazimuth beam width) of an antenna which approaches the length of theflight path over which the synthetic array is formed.

The required phase corrections associated with the data forming asynthetic array compensate for two-way range (R) variations, such asillustrated in FIG. 2. These range variations include a quadratic termapproximately equal to twice the distance between straight-line flightpath 12, and a theoretical semicircular constant range path (arc) 19centered on a given ground point scatterer 21. The magnitude of the Δsshown in FIG. 2 depicts the one-way range variation and have beenexaggerated to better illustrate the variation over the array length.For a given point along the flight path, the required phase correction(in radians) resulting from the two-way path length variation is Φwhere: ##EQU1## where λ is the radiated wavelength. For purposes ofexplanation, Δ may be approximated as Δ=C² /2R; where C is the distancealong the flight path measured relative to the array center and R is therange to a given point scatterer when the illuminating beam is broadsidethereto. A different point scatterer displaced in azimuth, such as point23, would have a constant range semicircular path (not shown) slightlydisplaced from arc 19; and hence the required phase corrections would beslightly different.

Hereinafter were discussed two techniques for applying the proper phasecorrection to the received radar data, i.e. the "line-by-line" and the"batch" processing techniques. A significant aspect of the subjectinvention is the method whereby the advantages of each of these twotechniques are realized while their respective limitations areminimized.

In the interest of clarity, a relatively simple example for the radartelescope mode of operation of the subject invention has beenillustrated in FIGS. 1, 1a and 3. The area 16 (FIG. 1) to be mappedencompasses a square 3,000 feet on a side and the total array length (L)in FIG. 3 is composed of 12 subarrays, indicated by reference numeral24--each of length L/12. Data gathered along each subarray is processedby a line-by-line technique to form a low azimuth resolution map foreach subarray. For example, the map of FIG. 1 could comprise 48 blocks,such as 26, with each block having a 60 foot azimuth resolution andcontaining 600 (5 feet resolution) cells in the range dimension, such ascell 31. One such low resolution map is formed for each subarray, suchas 24 of FIG. 3. For the selected example, twelve maps are formed in atime sequence to provide twelve angle diversity coherent "looks" at thetarget area. The resultant data for corresponding points from each ofthe low resolution maps (corresponding azimuth block and range cell) areprocessed to form twelve, 5 foot azimuth resolution cells, such as cell28 of FIG. 1a, for each of the blocks.

FIG. 3 depicts three of the 48 low azimuth resolution blocks, sometimeshereinafter referred to as columns of resolution blocks, 26a, 26b and26c in greater detail. Each of these low azimuth resolution blocks arefurther broken down into a plurality of subblocks in the rangedimension. The significance of the range dimension subgrouping isrelated to the phase variation of the received signals as a function ofthe swath width (across track dimension). In high resolution mappingapplications compensation for this phenomena must be applied at periodicrange intervals defined by the allowable defocusing effect assigned tothis error contributor. Such range intervals are sometimes referred toas the system's "depth of focus", which for example could be 500 feet.It can be shown that adequate focusing is obtained a distance Z=2d² /λaway from the range of best focus. For d=5 ft., λ=0.1 ft. The depth offocus 2Z=1000 ft. In the illustrated embodiment the length of the rangesubblocks are selected to be approximately equal to the system's depthof focus. For the mapping situation of FIG. 1, there would be six such500 foot range subblocks in each azimuth resolution block. Only threesuch range subblocks are shown for each azimuth block in FIG. 3 topreserve the clarity of the drawing.

As will be explained in detail hereinafter, the phase center 27 of eachof the subblocks are controlled to lie along parallel lines so that thetotal area covered by the subblocks approximates a square, not akeystone. Also the azimuth pointing direction of the individualsubblocks is programmed as a function of the position of the aircraftalong the array. Hence, the loq azimuth resolution maps may be formedsuch that the 48 azimuth by 6 range subblocks form a square 3,000 feeton a side with little distortion.

MECHANIZATION

Referring now primarily to FIG. 4, a synchronization and control unit 50applies synchronizing pulses to a conventional coherent pulsetransmitter 52 which in response thereto provides coherent output pulsesof RF energy 54 (see waveform 56 of FIG. 9a). Synchronization andcontrol devices suitable for unit 50 are well known in the art and maybe mechanized, for example, by a high frequency stable master oscillatorand associated circuitry (not shown) for multiplying the oscillator'sfrequency to provide RF signals required by transmitter 52 and localoscillator unit 62. Additionally the unit 50 may include circuitry forcounting down the frequency of the master oscillator to the pulserepetition frequency (PRF); and circuitry for producing range gatepulses in response to control signals from an array center computer 34.

The RF output pulses from transmitter 52 are applied through a duplexer58 to antenna 14 from whence they are radiated as illuminating beam 18(FIG. 1). Antenna 14 is positioned in response to antenna control unit36, which in turn is controlled by computer 34, such that theilluminating beam 18 is centered on the area to be mapped.

The reflected energy received from scatterers within antenna pattern 18is applied from antenna 14 through duplexer 58 to mixer 60. The RFreference signal applied from L.O. unit 62 varies as a function of therelative position of the aircraft so as to maintain the relative phaseof the signal received from the center of the mapped areaconstant--i.e., the doppler frequency of the center of the beam is"tracked out". The tracking of the phase history of the center of thebeam may be performed by several techniques one of which is illustratedin FIG. 4. As there shown, computer 34 computes the doppler frequency ofthe beam center from the flight geometry and antenna coordinates; andcontrols a voltage controlled oscillator 63 such that the frequency ofthe signal from the beam center is maintained constant at the output ofmixer 60. L.O. unit 62 may, for example, comprise a mixer (not shown)for forming the signal (f_(c) -30 Mhz)+f_(t) where: (f_(c) -30 Mhz) isthe transmitted frequency less the IF frequency, applied from unit 50;and f_(t) is the doppler frequency at the center of the beam, appliedfrom voltage controlled oscillator 63. Also, computer 34 computes theminimum and maximum ranges, on each range sweep (PRF interval), whichencompass the area to be mapped--i.e. the range intervals of area 16 ofFIG. 1. The implementation of the function described hereinabove forcomputer 34 is shown in greater detail in co-pending application Ser.No. 736,928, filed June 4, 1969 by Frederick C. Williams and assigned tothe assignee of the subject application. These maximum/minimum rangevalues are applied to synchronization and control unit 50 which in turnprovides range gate signals ("shift in" control signals) on an outputlead 74, and "shift out" control signals on an output lead 90. Therelative timing of the "shift in" and "shift out" control signals forseveral PRF intervals are shown in waveforms 40 and 42, respectively, ofFIG. 9a. The change in the range of gating signals 44 during theadjacent PRF intervals have been exaggerated to illustrate the pointthat the relative range of these signals is programmed by computer 34 sothat data from the area to be mapped is sampled as the aircraft movesalong the array path.

The output signals from mixers 60 phase compensated to the center of thearray are applied to, and amplified by, an IF amplifier 64; and are thenphase detected in a phase detector 68 against an IF reference signalapplied on a lead 66. The IF reference signal is also produced bysynchronization and control unit 50 as a function of the basic frequencyreference of the system. If the input signal to detector 68 isrepresented by a vector of amplifier A with a phase B relative to thephase of reference signal 66 (arbitrarily established as a phasestandard), then the output signal therefrom may be represented by thequantity A cos B which is sometimes hereinafter designated "I" forin-phase video.

Similarly, the signal from amplifier 64 is also applied to a quadraturephase detector 70; and the signal 66, after being phased shifted 90degrees by a phase shifter 72, is applied as the reference thereto.Hence, the output signal of phase detector 70 is translated 90 degreesfrom that of detector 68 and may be represented by the quantity A sinB--which quantity is sometimes hereinafter designated "Q" for quadraturevideo.

As mentioned above, the range interval to be mapped is determined by the"shift-in-control" signal applied on lead 74 from synchronization andcontrol unit 50. These pulses are applied to and control the samplingoperation of analog to digital (A/D) converters 77 and 78, as well asthe operation of buffer storage units 80 and 82. The minimum/maximumrange gates may be mechanized by a counter (not shown) which controls aflip-flop circuit (not shown) such that the flip-flop is set when thecounter counts a number of master oscillator pulses, as determined bycomputer 34, corresponding to the start of the mapped area. Theflip-flop is reset when a second count, also determined by computer 34,corresponding to the end of the mapped area has been reached. Theshift-in-control signal may then be formed by combining theminimum/maximum range gate signal with sampling clock pulses. Therepetition frequency of the clock pulses is determined by the desiredrange resolution, i.e., the length of the individual range zones orsegments to be processed, (5 feet in the illustrated embodiment). Uponthe application of the shift-in-control signals analog-to-digitalconverter units 77 and 78 sample the inphase and quadrature videosignals applied from detectors 68 and 70 respectively. Units 77 and 78convert the video signals to digital words of the desired precision,e.g. each word could comprise 8 bits including a sign bit.

The digital words representative of the value of the inphase andquadrature signals are applied from the converter units on compositeleads 79 and 81 to buffer storage units 80 and 82, respectively.Relative to FIG. 4, the term "composite lead" means that, although aseparate lead for each data bit is utilized, for the sake of clarity ofthe drawing only one lead per data channel is shown. It will beunderstood that in the discussion of the digital circuits hereinafterpresented that composite leads are employed where appropriate. It shouldalso be noted that, by appropriate data reformating, the parallel bitreadout could be converted into a high speed serial bit readout so thatonly one wire could be utilized. However, such high speeds are rarelypractical in high resolution processors.

The inphase and quadrature binary data words are shifted out of thebuffer units 80 and 82 to presum units 84 and 86 on composite leads 88and 91 respectively. The buffer and presum units are controlled inresponse to "shift out" control signals (waveform 42 of FIG. 9a) appliedon a lead 90 from the synchronization and control unit 50.

A mechanization suitable for presum units 84 and 86 is shown in FIG. 5and will be explained relative to unit 84. The digital data words areshifted out of buffer storage unit 80 on lead 88 and are applied to afirst input circuit of a summer 94, the output circuit of which iscoupled to a switch 96. The switch 96 has a first output circuit coupledon a lead 98 to a second buffer storage unit 51 and a second outputcircuit coupled to a presum register 102. The output signal of presumregister 102 is applied to the second input circuit of summer 94.

The operation of units 84 and 86 may be explained in terms of theexample of FIG. 3, and arbitrarily selected system parameters. For a PRFof 800, a range resolution of 5 feet and a presum ratio of 4 to 1,600range samples are fed to buffer units 80 and 82 in 6.1 μsec., every1/800 of a sec. The buffer units 80 and 82 will read out the storedsamples during the interpulse period (1250-6.1 μsec.) in response to theshift out control signals. The presum ratio is chosen so thatapproximately one sequence of 600 output pulses enters the processor 100each d feet of aircraft travel. For an azimuth resolution of d=5 feetand an aircraft speed of 900 ft./sec., a presum ratio of about 4 to 1 isappropriate. For this presum ratio the switch 96 (FIG. 5) connects theoutput of the summer 94 to the 600 word presum register 102 for threeconsecutive transmission pulse intervals; and connects the summer outputcircuit to the input of second buffer unit 51 during the fourthinterpulse interval. This sequence is repeated every four transmissionperiods. Hence, the data from buffer units 80 and 82 is read into bufferunits 51 and 53, respectively, at a rate equal to one-fourth thetransmission repetition frequency. Hence, the function performed by thepresummers is to process the received data to provide the unfocused sumof a number of consecutive returns associated with each range interval,so as to reduce the processing rate required by processor 100.

Buffer units 51 and 53 receive the bursts (600 words per burst) of data(inphase and quadrature) during a 1250 μsec. interval every 5milliseconds and applies this data on leads 104I and 104Q, respectively,to the 48 channels (FIG. 6) of processor 100 at a uniform rate over the5 millisecond interval (see FIG. 9b). Buffer units 51 and 53 arecontrolled by processor clock pulses, synchronized to the system of FIG.4, applied on lead 71 from synchronization and control unit 50.

One preferred embodiment of processor 100 is shown in a simplified blockdiagram form in FIG. 6 as comprising 48 parallel processing channels.Each channel includes an arithmetic unit 110 and a filter unit 112. InFIG. 6, the arithmetic and filter units associated with channel one aredesignated 110-1 and 112-1, respectively; and the elements of the otherchannels are correspondingly assigned a channel identification numberfollowing the element reference numeral. The operation and mechanizationof each of the processing channels are identical, with each channelproviding output signals to display unit 114. The output signals fromeach of the channels define the high azimuth resolution characteristicsof one azimuth block 26 (FIG. 1). Considering channel one, for example,during each one of the 12 subarray flight segments (see waveform 103 ofFIG. 9c), 100 presum input data bursts of 600 words (range cells) perburst at a repetition rate of 200 bursts per second are applied thereto,in parallel with the other 47 processing channels. It is recalled thatthe presum input video signals have been range and phase corrected tothe center 38 of mapped area 16 by the shift in control signals and thelocal oscillator reference signals, respectively, under the control ofcomputer 34 (FIG. 4). Unit 110 provides the necessary range correctionto cause the center of the subblocks, such as 27a shown in FIG. 3, tolie on straight lines rather than on a cord of a constant range circle.Since the input data is range correlated to the center of the map area,only one range correction is required by unit 110 for each channel, foreach of the subarray segments.

Unit 110 also applies the appropriate phase correction to each inputdata word (range cell); and accumulates the phase corrected signals foreach range cell during subsequent input signal bursts of each subarrayto provide focused output signals for each of the range cells sometimeshereinafter referred to as resolution cells. These output signalsrepresent the value of the signals received during the subarray periodfrom each range bin focused on the center of the corresponding 60 footazimuth resolution block (26 of FIG. 1). Hence, in the disclosedembodiment during each subarray period units 110 of the 48 parallelprocessing channels provide focused output signals for 48 columns ofresolution cells in the azimuth dimension, and 600 rolls of resolutioncells in the range dimension; and as is explained above and as shown inFIG. 3, these resolution cells are substantially, rectangularlyoriented. At the end of each subarray time period 600 output words fromeach azimuth processing channel, sometimes hereinafter collectivelyreferred to as a set of imagery data, are provided from arithmetic unit110 and applied to filter unit 112. Each of these words are indicativeof the characteristics of a mapped area 60 feet in azimuth and 5 feet inrange resolution. Filter unit 112 stores, processes, and stores theresidues of the processed signals from corresponding resolution cells(same azimuth and range location) from each low resolution mapssequentially formed during each of the 12 subarrays. At the end of thetotal array (subarray number 12 having been completed) the filter bank112 provides output signals defining the high resolution map--such asresolution elements 28 of FIG. 1a. Each of the 48 processing channelsprovide 12 output signals, sometimes hereinafter collectively referredto as a subset of high resolution synthetic array imagery data, for eachof the 600 range resolution cells. Hence, 48×12×600 output signalsindicative of the map characteristics of the illuminated ground area areprovided to display unit 114. Unit 114 displays the mapped area with 5foot resolution in both range and azimuth and without noticeable"keystone" problems.

One of the processing channels of FIG. 6 is shown in greater detail inFIG. 7. As there shown, the input data to the processor 100 (inphase andquadrature) is applied on composite lead 104 to an interpolator 116,sometimes hereinafter referred to as delay means, which comprises ashift register 118 and a switching network 120. Shift register 118 may,for example, be eight words in length. Switching network 120 includesthe necessary switching and logic circuits for coupling a selected oneof the output taps of the shift register to a multiplicand inputterminal of a complex multiplier unit 122, in response to a digital codeapplied to the switching network from a coefficient storage unit 124.Interpolator 116 provides the necessary range corrections, sometimeshereinafter referred to as "relative time delay," for each of the 12subarrays to place the center of each subblock (FIG. 3) on a straightline rather than on a circle of constant range. Therefore, 12 values ofrange coefficients (τ) must be stored in the coefficient storage unit124. These values may be computed by a computer 126 from parameters ofthe flight geometry supplied from auxiliary systems (not shown) and/orfrom information manually entered into computer 126. It is noted thatboth the inphase and quadrature components of the signals are processedby unit 100, although only a single processing path is shown in FIGS. 6and 7 in order to maintain the clarity of the drawings. It is understoodthat all digital units are appropriate complex devices for performingthe indicated operations on the inphase and quadrature signalcomponents. For example, where the data words applied through switchingnetwork 120 are designated x and the desired coefficient multipliers bythe complex number A, the complex multiplication Ax applies theappropriate phase correction and amplitude scaling of the input datasignals to provide the desired focusing and "sidelobe" levels of thesynthetic subarray. This complex multiplication function may be moreclearly visualized by recalling that the product of two complex numbers,A_(I) +jA_(Q) and D_(I) +j D_(Q), is (A_(I) D_(I) -A_(Q) D_(Q))+j(A_(Q)D_(I) +D_(I) A_(Q)) where D_(I) and D_(Q) are the inphase and quadraturecomponent terms of the data words and A_(I) and A_(Q) are the inphaseand quadrature terms of the complex multipliers. (A_(I) D_(I) -A_(Q)D_(Q)) and (A_(Q) D_(I) +A_(I) D_(Q)) are the inphase and quadratureterms respectively of the complex product of these two complex numbers.Means for mechanizing the above product terms are known in the art--suchas those illustrated in copending application Ser. No. 73,470, filedSept. 18, 1970, by Frederick C. Williams, entitled "PolyphaseEncoding-Decoding System", and assigned to the assignee of the subjectapplication.

The output signal from multiplier 122 is combined with the signal at theoutput of shift register 128 in a summation unit 130 and the resultantsum signal is normally applied through a switching circuit 132 to theinput of shift register 128.

The shift register 128, sometimes hereinafter referred to as a serialdata storage device, which may be 600 words in length, functions tostore the partial sum for each of the 600 range cells as each rangecell, for the azimuth block associated with the channel, is focusedduring a subarray time period. For each of the subblocks, such as block29 of FIG. 3, the same focusing coefficient is applied to each inputdata burst. For the parameters presented above, the total time to fly 12subarrays is 6 seconds, and the time of a single subarray is T/12=0.5seconds. At an input data rate of 200 presumed video bursts per second,there are 100 data bursts, each containing 600 words (each wordassociated with a range resolution cell) for each subarray.

After range interpolation within unit 116, the data from one subarray ismultiplied by a 100 constants (A) as a function of phase focusing froman azimuth point of view. To compensate for the depth of focusconsiderations the coefficients A also are a function of the rangesubblock being processed. Hence, the coefficients to multiplier 122 aredesignated A_(b-f) where first subscript identifies the number of thepresumed data burst, and the second subscript identifies the rangesubblock. For example, for the 100 bursts of data associated with anyparticular subarray these are 100 coefficients associated with therespective bursts sequences for the first range subblock; a new set of100 coefficients associated with the second range subblock, etc., for atotal of 600 focusing coefficients per subarray.

The 600 partial sums for each 60 foot azimuth block (each processingchannel) are stored in shift register 128 and are circulated through thesummation circuit 100 times during the formation of a subarray. Thesesignals correspond to 3,000 feet of range coverage at 5 feet rangeresolution.

With reference to FIG. 8, the difference (δR) between the distance fromthe center of the synthetic array to the center of the mapped area 16(R_(o), θ), and the distance from the points vt along the syntheticarray to a point located at a distance (x, y), away from the center ofthe area being mapped can be written to terms of the third order as:##EQU2## The inphase and quadrature terms for multiplier coefficients Aare Cos δφ and Sin δφ, respectively; where δφ is equal to 4πδR/λ. Thesemultiplication values are computed for each (R_(o), θ) corresponding tothe center of the synthetic array (at 6 second intervals); for each xvalue (the center of each azimuth block, 48 values); and y value, (eachsubblock in the range dimension, 6 values) for each presumed data burst(200 burst per second--600 words per burst). Programming computer 126for this computation could include incrementing in x, y and t and doubleincrementing to obtain x² and t² --techniques which are well known inthe computer programming art.

At the end of the first subarray, the signals from summation circuit 130are indicative of the low azimuth resolution map (60 foot resolution)for the azimuth block associated with a particular channel. Means forprocessing this data and the data generated during the subsequentsubarrays, to form the high resolution map (see FIG. 1a) will now beconsidered. Again, referring primarily to FIG. 7, at the end of thefirst subarray (the 100th data burst for the selected parameters) switch132 applies the output data from summation circuit 130 through a switch134 to a shift register 136, instead of returning this data to shiftregister 128. At this time, at the end of a subarray, the low resolutionmap had been formed by the phase rotation and accumulation operationsperformed within multiplication unit 122, shift register 128 andsummation circuit 130. This data is further processed and stored, andcombined with data from subsequent subarrays to provide the highresolution map.

During the second subarray (the first filter processing time P₁ of FIG.9c) the low azimuth resolution data formed during the first subarray andstored in the 600 stages (one for each range resolution element) ofshift register 136 is circulated 12 times. During each circulation theproper multiplier values (B) are applied from a coefficient storage unit138 to a complex multiplier unit 140. The output signals from multiplier140 are used to digital frequency synthesis of the correspondingportions of the high azimuth resolution maps from the 12 sequentiallyformed low azimuth resolution maps. During each of the 12 recirculationsof shift register 136, the proper phase rotation and amplitude weighting(complex multiplication by B) is applied to the data and it is thencoupled through a summation unit 142 to a memory device 144.

The memory device 144 which may be of the random access memory type, hasits addressing format programmed such that during the second subarray(first filter processing period P₁) 12 phase shifted values (multipliedby different B values) for each range resolution cell are stored withinthe memory. During the following subarray (filter processing period P₂)the twelve stored value for each of the resolution cells aresequentially retrieved from the memory and summed in summation circuit142 with data from a corresponding resolution cell and circulationcycles to form first residue signals. These first residues, sometimeshereinafter referred to as partial sum signals, are then stored inmemory 144 and during the following subarray time period (period P₃) aresequentially retrieved and combined with data from a correspondingresolution cell and circulation cycle from subarray 3 to form secondresidue signals.

The above just described process mechanizes by a Fourier Transformtechnique, the equivalent of a bank of equally spaced adjacent digitalfilters, which encompass the doppler frequencies defining the differentcells (28 of FIG. 1a) for the high resolution map. For example,considering the first range resolution cell of a low resolution azimuthblock such as block 26 (FIG. 1), if X (k) for k equal to 1, 2, 3, . . .N, is the value of the low resolution signal formed during each of the Nsubarray time periods then: ##EQU3## is the value of the ith filteroutput signal, where i also is equal 1, 2, 3, . . . N. g(k)e^(j2)πik/Nis the B multiplication coefficient of multiplier 140 discussed above.The term g(k) is an amplitude weighting function which may be includedin each of the multiplication operations to obtain the desired responseshape for the filter bank outputs. For example g_(k) could be of atruncated gaussian shape centered at the center subarray data point.

Therefore, on the last twelve circulations of the data in register 136following subarray number 12 (filter processing period P₁₂), outputsignals, indicative of the high azimuth resolution cells (28 of FIG.1a), are formed in summation circuit 142. These signals may be stored inmemory 144 as well as applied on an output lead 146 to display unit 114(FIG. 6). For the selected parameters, there will be 12 output signalsfor each of the 600 range resolution cells for each of the processinngchannels. Hence, data defining 600×48 (channels)×12 individualresolution cells, each 5 feet by 5 feet are stored in the memory 144and/or applied to the display unit 114. The multiplication coefficientsapplied to multiplier 140 from coefficient storage unit 138 could beprecomputed and manually entered in unit 138 or preferably they may becomputed by computer 126 in accordance with Equation (2). These Bcoefficients applied to multiplier 140 consists of 144 values (12circulations of each of the 12 subarrays) for each of the 6 rangesubblocks and they are common to all 48 parallel processing channels.

OPERATION

To summarize the operation of the processor of FIG. 7, interpolator 116is controlled by computer 126 (τ coefficients) so as to cause thecenters of the resolution subblocks (FIG. 3) to lie on a straight linerather than along a cord of a circle of constant range. The line-by-lineprocessing portion of each channel, such as units 122, 130, 132 and 128,form a low azimuth resolution map for each of the 600 range cells of theassociated low azimuth resolution block. The digital frequency synthesisportion of the processor channel shown in FIG. 7, such as unit 134, 136,140, 142 and 144, respond to the data from corresponding parts of thelow azimuth resolution maps formed during each of the subarrays toprovide a high resolution map.

The timing sequence of the operation of processor 100 will now beexplained by outlining the steps in the formation of one high resolutionmap. The multiplier coefficients applied to complex multiplier unit 122are expressed as A_(b-f) where the first subscript identifies the databurst number (1 through 1200) and the second subscript the rangesubblock, sometimes hereinafter referred to as a range subgroup (1through 6). The received data from interpolator 116 is designatedx_(b-r) where the first subscript is the burst number and the secondsubscript corresponds to the range cell (1 through 600). The data pulsesof the first burst (600 range gated returns from the first transmissionperiod) are focused and amplitude weighted within multiplier 122, andthe output signals therefrom may be written in a time sequence as:

    A.sub.1-1 x.sub.1-1, A.sub.1-1 x.sub.1-2 . . . A.sub.1-1 x.sub.1-100, A.sub.1-2 x.sub.1-101 . . . A.sub.1-6 x.sub.1-600         (3)

It is noted that every 100 range intervals the value of the multipliercoefficients are changed to correct for the depth of focus effectsdiscussed previously. This sequence of signals is applied frommultiplier 122 by means of switch 132 to shift register 128. On thesecond input data burst, multiplier 122 provides output signals:

    A.sub.2-1 x.sub.2-1, A.sub.2-1 x.sub.2-2 . . . A.sub.2-1 x.sub.2-100, A.sub.2-2 x.sub.2-101 . . . A.sub.2-6 x.sub.2-600         (4)

These signals are combined in summation circuit 130 with the signalsstored in shift registers 128 and the sum thereof are applied throughswitch 132 to shift register 128. At the end of the second data burstthe signals stored in shift register 128 are: ##EQU4## On the 100thburst, the output of multiplier 122 is a sequence of signals:

    A.sub.100-1 x.sub.100-1, A.sub.100-1 x.sub.100-2, . . . A.sub.100-1 x.sub.100-100, A.sub.100-2 x.sub.100-101 . . . A.sub.100-6 x.sub.100-600 ( 6)

and the output of summation circuit 130 on burst 100 is:

    X.sub.1-1, X.sub.1-2 . . . X.sub.1-100, X.sub.1-101 . . . X.sub.1-600 (7)

where the first subscript of the "X" define the subarray; the secondsubscript the associated range cell and the notation ##EQU5## Forexample, the output signal X₁₋₁ from summation circuit 130 on the firstprocessing period (range cell) of the 100 burst is:

    A.sub.1-1 x.sub.1-1 +A.sub.2-1 x.sub.2-1 +A.sub.3-1 x.sub.3-1 +. . . A.sub.100-1 x.sub.100-1                                   (8)

On the 100th burst which is the end of the first subarray time period,switch 132 is operated to cause the data from the summation circuit 130to be shifted into shift register 136 instead of into shift register128. Hence, during the 100th burst, shift register 128 is emptied.

In a similar manner, on burst 101 the output signals from multiplier 122are:

    A.sub.101-1 x.sub.101-1, A.sub.101-1 x.sub.101-2, . . . A.sub.101-1 x.sub.101-100, A.sub.101-2 x.sub.101-101, . . . A.sub.101-6 x.sub.101-600 ;                                                         (9)

and as described for the first subarray, the partial sum of the focuseddata for each range cell is accumulated during the second subarray suchthat on the 200 burst the output of summation circuit 130 is:

    X.sub.2-1, X.sub.2-2 . . . X.sub.2-100, X.sub.2-101 . . . X.sub.2-600 (10)

Also on the 200th burst which is the end of the second subarray timeperiod, switch 132 is operated to cause the data at the output ofsummation circuit 130 to be shifted into shift register 136 instead ofinto shift register 128.

This just described operation is repeated for every successive subarraysuch that on burst 1200 the output signals from summation circuit 130are:

    X.sub.12-1, X.sub.12-2 . . . X.sub.12-100, X.sub.12-101 . . . X.sub.12-600 (11)

Reference is now directed to the digital frequency synthesis portion(units 134, 136, 138, 140, 142 and 144) of the processor channel shownin FIG. 7. The following description explaining how effectively 12filters are formed for each of the 600 range intervals. On burst 100 thesignals X_(1-r) are shifted into shift register 136 and during burst 101through 199 (filter processinng time P₁) these signals are recirculated12 times in the loop which includes switch 134 and register 136. Thesignals stored in the register 136 represent the value of the resolutioncells (600 range intervals) for the low azimuth resolution map formedduring the last preceding subarray. During each circulation of this datait is multiplied in multiplier 140 by a different coefficient (B) asdefined by Equation 2. Due to changes in the doppler frequencydifferential across a low azimuth block (26) as a function of range, itmay be desirable to adjust the g(k) weighting function to optimizecoverage of the synthesized filter bank each range subblock. Hence the Bcoefficients may be varied as a function of the range subblock, however,to clarify the explanation, this variable (B as a function of R) isignored in the following analysis. During the first circulation of thedata, the terms:

    B.sub.1-1 X.sub.1-1, B.sub.1-1 X.sub.1-2 . . . B.sub.1-1 X.sub.1-600 (12)

are formed and stored in memory 144. The first subscript of thecoefficient B identifies the subarray number; and the second subscriptidentifies the filter number (also the circulation cycle). On the secondcirculation of the data (X₁) formed during the first subarray, theterms:

    B.sub.1-2 X.sub.1-1, B.sub.1-2 X.sub.1-2 . . . B.sub.1-2 X.sub.1-600 (13)

are formed and stored in the memory 144. This process continues untilthe 12th circulation wherein terms:

    B.sub.1-12 X.sub.1-1, B.sub.1-12 X.sub.1-2 . . . B.sub.1-12 X.sub.1-600 (14)

are formed and stored in memory 144.

on burst 200, switch 134 opens the "feedback" loop 135 around shiftregister 136. During burst 200, the old data (from the first subarray)is "dumped" and new data from the second subarray: X₂₋₁, X₂₋₂ . . .X₂₋₆₀₀, is shifted into register 136 through switches 132 and 134.During burst 201 through 299 (filter processing time P₂), the new dataassociated with subarray number 2 is circulated 12 times in a mannersimilar to that explained above for subarray number 1. However, now thedata corresponding to each filter (circulation cycles), for each rangebit, that was stored in memory 144 during the last filter processingperiod is recalled from memory 144 during the appropriate cycles suchthat the partial sums required to produce the effect of 12 filters maybe accumulated. For example, during the first circulation of period P₂the terms ##EQU6## are formed and stored in memory 144. Considering thefirst term of Equation (15), the quantity B₁₋₁ X₁₋₁ was formed duringthe processing period P₁ and was recalled from memory after the termB₂₋₁ X₂₋₁ were formed by multiplier 140 during the first processing stepof the first cycle of period P₂. In a similar manner the terms for eachrange cell of the data for circulation 1 are formed as indicated byEquation (15) and similar type series of terms are formed duringcirculations 2 through 12. During circulation 12 the following sequenceof terms would be stored in memory 144: ##EQU7##

Twelve partial sums for each of the 600 range cells are formed duringeach filter processing time period in the same manner as just outlined.For example, during the time period of burst 1201 to 1299 (filterprocessing period P₁₂), on the first circulation of register 136, theterms:

    Y.sub.1-1, Y.sub.1-2, Y.sub.1-2 . . . Y.sub.1-600          (17)

are stored in memory 144. The term Y₁₋₁ would be the completed outputsignal for the first filter of the first range cell in accordance withEquation (2), and represents the value of one high resolution cell (5feet resolution in range and azimuth) of the completed map. The termY₁₋₂ would represent the value associated with the first filter of thesecond range cell and so forth for the remaining range cells, with theterm Y₁₋₆₀₀ being the value for the first azimuth high resolution celland the range cell 600. In a like manner during subsequent circulationsthe high resolution map is completed term by term with the last seriesof term

    Y.sub.12-1, Y.sub.12-2, Y.sub.12-3 . . . Y.sub.12-600      (18)

completing the map. The term Y₁₂₋₁ is the value of the 12th filter(azimuth resolution cell) for the first range cell.

Thus there has been described a new and efficient method and apparatusfor processing radar data to provide high resolution synthetic arraydata in a format readily adapted for display.

We claim:
 1. A system for processing radar data received from a selectedarea of a surface illuminated by a radar beam during a plurality ofsubarray flight path segments so as to produce high resolution syntheticarray data, said system comprising:first processor means for adjustingthe relative time delay and phase of the radar data received during eachsubarray flight path segment and for summing the resultant signalsassociated with the same resolution cell in said area to consecutivelyprovide sets of imagery data, with each set corresponding to the samegroup of substantially rectangularly oriented resolution cells butderived from radar data from different subarray flight path segments;and second processor means for filter processing the data ofcorresponding resolution cells from each of said sets of imagery data toproduce a subset of high resolution synthetic array imagery data foreach resolution cell, with each subset corresponding to a plurality ofresolution elements.
 2. The processor of claim 1 wherein said selectedgroup of resolution cells has columns of resolution cells approximatelyparallel to a first axis and rolls of resolution cells approximatelyparallel to a second substantially orthogonal axis, and said firstprocessor means includes a plurality of first parallel processingchannels equal in number to the number of said columns, with eachchannel providing the imagery data for a different one of said columnsand including:delay means for delaying the received radar data as afunction of the subarray flight path segment of the radar data beingprocessed; first multiplier means coupled to said delay means for phaseand amplitude adjusting the output signals therefrom as a function ofthe motion along said flight path of the source of said radar beam;thereby electronically focusing the received radar data at theapproximate center of the associated column; and accumulator-circulatormeans for sequentially forming from the data of each subarray flightpath segment the sum of the phase-amplitude adjusted data for each cellin the associated column, to provide said imagery data for theresolution cells of said associated column.
 3. The processor of claim 2wherein said second processor means includes a plurality of secondparallel processing channels with each second parallel processingchannel coupled to a different one of said first parallel processingchannels and including:second multiplier means coupled to theaccumulator-circulator means of the associated first parallel processingchannel for multiplying the applied imagery data for the resolutioncells of the associated column by a plurality of complex coefficients toproduce a plurality of product signals; and means for summing selectedgroups of said product signals associated with all of the subarrays, toprovide said subset of high resolution synthetic array imagery data foreach resolution cell.
 4. The processor of claim 3 wherein said secondmultiplier means includes means for circulating the imagery data for theassociated column of resolution cells from each subarray flight pathsegment a number of times equal to the number of subarray flight pathsegments, and means for multiplying said data during each circulation bya different complex coefficient.
 5. The processor of claim 4 whereinsaid means for summing the product signals includes means for formingand storing partial sum signals during each circulation cycle, and meansfor updating these stored partial sum signals as a function of theassociated product signal for the same resolution cell formed during thenext circulation cycle.
 6. The system of claim 1 further comprisingcompensation means for adjusting the frequency of the radar data so asto maintain the frequency of the data received from the central portionof the illuminating beam substantially constant.
 7. The system of claim2 wherein a plurality of range sweeps of data are sequentially receivedeach subarray flight path segment, with each range sweep having aplurality of range zones and each range zone being associated with adifferent one of said rolls of resolution cells, and wherein selectedgroups of adjacent rolls of resolution cells define depth of focus rangesubgroups and said first multiplier means includes multipliercoefficient control means for varying the phase and amplitudeadjustments as a function of the range sweep and range subgroup to causethe center of said range subgroups to be approximately linearly disposedalong said second axis.
 8. The system of claim 7 wherein saidaccumulator-circulation means includes:a summation unit having a firstinput circuit coupled to the output of said first multiplier means, asecond input circuit and an output circuit; first switching means havingan input circuit coupled to the output circuit of said summation unitand having first and second output circuits, for coupling its inputcircuit to its first output circuit during the processing of each ofsaid range sweeps except the last range sweep of each subarray, and forcoupling its input circuit to its second output circuit during theprocessing of said last range sweep of each subarray; a first serialdata storage device having an input circuit coupled to the first outputcircuit of said first switching means, and an output circuit coupled tosaid second input circuit of said summation unit; and means for couplingsaid second output circuit of said first switching means to said secondprocessor means; whereby the set of imaging data of the associatedchannel for each subarray flight path segment is accumulated in the loopincluding said summation unit, said first switching means and the firstserial data storage device, and the set of imagery data is then coupledthrough the second output circuit of said first switching means to saidsecond processor means.
 9. The system of claim 8 wherein said secondprocessor means includes a second switching means having a first inputcircuit coupled to the second output circuit of said first switchingmeans a second input circuit and an output circuit, for coupling itsfirst input circuit to its output circuit during the time period data isapplied to said second output circuit of said first switching means andfor coupling its second input circuit to its output circuit during othertime periods;a second data serial storage device having an input circuitcoupled to the output of said second switching means and having anoutput circuit coupled to said second input circuit of said secondswitching means; control means coupled to said second data serialstorage device for causing the data applied from said first switchingmeans to be circulated through said second data serial storage device anumber of times equal to the number of subarrays; second multipliermeans for multiplying the output signal of said second storage deviceduring each circulation by a different complex coefficient; a secondsummation unit having a first input circuit coupled to said secondmultiplier means, a second input circuit and an output circuit; andstorage means having an input circuit coupled to the output of saidsecond summation unit, a first output circuit coupled to said secondinput of said second summation unit and a second output; whereby aplurality of partial sum signals associated with each resolutioncell-complex coefficient combination is stored by said storge meansduring each circulation cycle and then updated as a function of thesignal of the corresponding combination provided by said secondmultiplication means during the next circulation period.
 10. The systemof claim 9 furthe comprising a display device coupled to said secondoutput circuit of said storage means.
 11. A system for producing highresolution synthetic array data comprising:radar means for transmittingand receiving energy to provide radar data during N subarray flight pathsegments; first processor means for providing N consecutive sets ofimagery data from said radar data, with each set of imagery datacorresponding to the same group of columns of resolution cells along afirst axis and rolls of resolution cells along a second axis; said firstprocessor means comprising a plurality of first parallel processingchannels with each processing channel providing imagery data associatedwith a different one of said columns and including delay means fordelaying the received radar data as a function of the subarray flightpath segment associated therewith to cause said rolls of resolutioncells to be substantially parallel; and focusing means for adjusting thephase of the received time delayed data and for summing the resultantphase adjusted signals associated with the same resolution cell to formsaid imagery data for the resolution cells of the associated column; andsecond processor means for filtering the data for correspondingresolution cells from each of said N sets of imagery data to produce asubset of high resolution synthetic array imagery data for eachresolution cell, with each subset corresponding to a plurality ofresolution elements.
 12. The system of claim 11 wherein said secondprocessor means includes a plurality of second parallel processingchannels with each said second processing channel coupled to anassociated one of said first parallel processing channels andincluding:first multiplication means for multiplying the imagery data byN complex coefficients to provide product signals; and means for summingselected groups of said product signals derived from the data from eachof said N subarrays to provide said subset of high resolution syntheticimagery data for each resolution cell.
 13. The system of claim 11wherein said focusing means includes:second multiplication means coupledto said delay means for phase and amplitude adjusting the time delayedradar data as a function of the motion along said flight path of saidradar means; and accumulator-circulator means for sequentially formingfrom the data from each subarray flight path segment, the sum of thephase-amplitude adjusted data corresponding to each resolution cell inthe associated column to provide said imagery data for the resolutioncells of said associated column.
 14. The system of claim 12 wherein saidfirst multiplier means includes means for circulating the imagery datafor the associated column of resolution cells N times, and means formultiplying said data during each circulation by a different complexcoefficient.
 15. The system of claim 12 wherein said means for summingthe product signals includes means for forming and storing partial sumsignals formed during each circulation cycle, and means for updating thestored partial sum signals as a function of the associated productsignal for the same resolution cell formed during the next circulationcycle.
 16. The system of claim 11 further comprising means for adjustingthe frequency of the received radar data to maintain the frequency ofthe radar data from the central portion of the radar beam substantiallyconstant.
 17. The method of producing high resolution synthetic arraydata comprising the steps of:transmitting and receiving energy toprovide radar data during N subarray flight path segments; adjusting therelative phase and time delay of the radar data received during eachsubarray flight path segment, and summing the resultant signalsassociated with the same resolution cell to provide N consecutive setsof imagery data with each set corresponding to the same group ofsubstantially rectangularly oriented resolution cells but differentsubarray flight path segments; and filtering the data of correspondingresolution cells from each of said N sets of imagery data to produce asubset of high resolution synthetic array imagery data for eachresolution cell.
 18. The method of claim 17 wherein said group ofresolution cells comprises approximately parallel columns of resolutioncells along a first axis and approximately parallel rolls of resolutioncells along a second orthogonal axis, and said step of adjusting therelative phase and time delay of the radar data and for summing theresultant signals includes the step of:delaying the received radar dataas a function of the subarray flight path segment associated therewith;multiplying the delayed received data by complex coefficients to adjustthe phase and amplitude thereof as a function of motion along saidflight path; and forming from the data from each subarray flight pathsegment the sum of the phase-amplitude adjusted data corresponding toeach cell to provide one set of said imagery data for each subarraysegment.
 19. The method of claim 17 wherein said step of filtering thedata of corresponding resolution cells from each of said N sets ofimagery data inclues multiplying the imagery data for each of theresolution cells by a plurality of complex coefficients to provideproduct signals; and summing selected groups of said product signalsderived from the data from each of said plurality of subarrays, toprovide said subset of high resolution synthetic array imagery data foreach resolution cell.
 20. The method of claim 19 wherein said step ofmultiplying the imagery data by a plurality of complex coefficientsincludes the steps of circulating the imagery data resulting from eachsubarray flight path segment N times and multiplying said data duringeach circulation by a different one of said complex coefficients. 21.The method of claim 19 wherein said step of summing the product signalsincludes forming and storing partial sum signals during each circulationcycle, and updating each stored partial sum signal as a function of theassociated product signal for the same resolution cell formed during thenext circulation cycle.